Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk. The escalating requirements for high areal recording density and increasingly smaller disk drives impose increasingly demanding requirements on thin film magnetic recording media in terms of coercivity, coercivity squareness, medium noise and narrow track recording performance. The requirements for efficiency and high productivity impose additional demands for minimizing downtime of production equipment. Considerable effort has been spent in recent years to produce magnetic recording media having higher recording densities and satisfying such demanding requirements, particularly for longitudinal recording.
A conventional longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al--Mg)-alloy, plated with a layer of amorphous nickel-phosphorous (NiP) . Alternative substrates include glass, glass-ceramic materials and graphite. Substrate 10 typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer 11, 11', a cobalt (Co)-base alloy magnetic layer 12, 12', a protective carbon overcoat 13, 13' and a lubricant topcoat 14, 14'. Cr underlayer 11, 11' can be applied as a composite comprising a plurality of sub-underlayers 11A, 11B, 11C, 11A', 11B', 11C'. Cr underlayer 11, 11', Co-base alloy magnetic layer 12, 12' and protective carbon overcoat 13, 13' are typically deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture, which is substantially reproduced on the disk surface.
In recent years, considerable effort has been expended to achieve high areal recording density. Among the recognized significant factors affecting recording density are magnetic remanence (Mr) , coercivity (Hc), coercivity squareness (S*), signal/noise ratio, and flying height, which is the distance at which a read/write head floats above the spinning disk. Prior approaches to achieve increased areal recording density for longitudinal recording involve the use of dual magnetic layers and multilayer media. For example, Teng et al., U.S. Pat. No. 5,462,796 discloses a magnetic recording medium comprising dual magnetic layers separated by an intermediate non-magnetic layer of Cr having a thickness of about 5 .ANG. to about 25 .ANG.. Lal et al., U.S. Pat. No. 5,432,012 discloses a magnetic recording medium comprising a gradient magnetic layer interposed between two magnetic layers. Furusawa et al., U.S. Pat. No. 4,950,548, disclose a magnetic recording medium comprising a plurality of chromium underlayers which exhibit a bow-like columnar structure, thereby enabling low modulation independent of the surface texture. In accordance with the disclosure of Furusawa et al., at least 2 layers, preferably 3 layers, of chromium are deposited each at a thickness of about 50-200 nm.
It is, of course, highly desirable to provide a magnetic recording medium with high coercivity and S*, and high signal-to-noise ratio. The various magnetic layers conventionally employed to produce magnetic recording media, typically Co or Co-alloys, exhibit differing properties such as different signal-to-noise ratios, coercivities and other magnetic properties. For example, certain cobalt alloys, such as cobalt-chromium-tantalum (Co--Cr--Ta) alloys, exhibit a superior signal-to-noise ratio vis-a-vis Co-alloys containing Cr and platinum (Pt) . However, the coercivity of Co--Cr--Ta alloys is undesirably low. It is possible to enhance the coercivity of magnetic alloys, such as Co--Cr--Ta alloys, by increasing the deposition temperature. However, high temperature deposition adversely affects various substrate materials, as by inducing crystallization of NiP. High temperature deposition also causes deformation of a texture provided on a substrate, such as protrusions formed by laser texturing.
There exists a need for a magnetic recording medium having a high signal-to-noise ratio, high S*, and high coercivity, and for a productive, efficient technique which enables the coercivity of a magnetic recording medium to be increased without resort to high temperature deposition of the magnetic layer and without substantially affecting the Mrt.